Metal connection member and method for chemical conversion treatment of metal connection member

ABSTRACT

A metal connection member including a Mg alloy member composed principally of magnesium, a partner member connected to the Mg alloy member, the partner member being composed principally of a metal nobler than magnesium, and a chemical conversion treatment film that covers a surface of the Mg alloy member and a surface of the partner member with covering a boundary between the Mg alloy member and the partner member.

TECHNICAL FIELD

The present invention relates to a metal connection member and a methodfor a chemical conversion treatment of a metal connection member.

The present application claims a priority to Japanese Patent ApplicationNo. 2017 154826 filed on Aug. 9, 2017, which is incorporated herein byreference in its entirety.

BACKGROUND ART

One of the known chemical conversion treatment methods in which achemical conversion treatment film is formed on the surface of analuminum (Al) member is, for example, the surface pretreatment ofaluminum described in NPL 1. NPL 1 discloses atechnique in which achromium free chemical conversion treatment liquid that includeszirconium (Zr) as a principal component is used.

CITATION LIST Non Patent Literature

NPL 1: Kiyotada Yasuhara, “Surface pretreatment of aluminum forcoatings”, light metals, The Japan Institute of Light Metals, 1990, Vol.40, No. 10, p. 753 to 760

SUMMARY OF INVENTION

A metal connection member according to the present disclosure includes:a Mg alloy member composed principally of magnesium; a partner memberconnected to the Mg alloy member, the partner member being composedprincipally of a metal nobler than magnesium; and a chemical conversiontreatment film that covers a surface of the Mg alloy member and asurface of the partner member with covering a boundary between the Mgalloy member and the partner member.

A method for a chemical conversion treatment of a metal connectionmember according to the present disclosure includes:

a preparation step of preparing a metal connection member, the metalconnection member including a Mg alloy member composed principally ofmagnesium and a partner member connected to the Mg alloy member, thepartner member being composed principally of a metal nobler thanmagnesium; and a chemical conversion treatment step of bringing both ofthe Mg alloy member and the partner member included in the metalconnection member into contact with a chemical conversion treatmentliquid in a collective manner to form a chemical conversion treatmentfilm on a surface of the metal connection member,

wherein the electric conductivity y of the chemical conversion treatmentliquid satisfies at least one of the relation formulae (a) and (b)below,

y≤0.0007 x ₁+14.0   (a)

y≤0.054 x ₂+14.2   (b)

-   -   where x₁ (Ω) is the charge transfer resistance of the Mg alloy        member in 1M of Na₂SO₄, x₂ (mass %) is the content of aluminum        in the Mg alloy member, and y (mS/cm) is the electric        conductivity of the chemical conversion treatment liquid.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross sectional view of a metal connection memberaccording to an embodiment.

FIG. 2 is a graph illustrating the relationship between the chargetransfer resistance x₁ (Ω) of a Mg alloy member and the electricconductivity y (mS/cm) of a chemical conversion treatment liquid.

FIG. 3 is a graph illustrating the relationship between the Al contentx₂ (mass %) in the Mg alloy member and the electric conductivity y(mS/cm) of the chemical conversion treatment liquid.

DESCRIPTION OF EMBODIMENTS Problems to be Solved by Present Invention

Al members are commonly connected to iron (Fe) members particularly inthe production of parts of automobiles or the like. Since subjecting anAl member and a Fe member individually to a chemical conversiontreatment requires multiple lines for the chemical conversion treatmentsof the members, the chemical conversion treatment may be performed bybringing an Al member and a Fe member into contact with a commonchemical conversion treatment liquid in a collective manner while the Almember is connected to the Fe member.

The application of magnesium (Mg) alloy members is being studied inorder to achieve weight reduction. However, there has not been developedany specific method in which a chemical conversion treatment of a Mgalloy, which is more base than Al, and a Fe member is performed bybringing the Mg alloy and the Fe member into contact with a commonchemical conversion treatment liquid in a collective manner as describedabove while the Mg alloy and the Fe member are connected to each other.Therefore, the development of such a chemical conversion treatmentmethod is anticipated.

Accordingly, it is an object to provide a metal connection member thatincludes a Mg alloy member, a partner member connected to the Mg alloymember, and a chemical conversion treatment film that covers the Mgalloy member and the partner member, the chemical conversion treatmentfilm having good adhesion and good corrosion resistance.

It is another object to provide a method for a chemical conversiontreatment of a metal connection member, the method enabling the chemicalconversion treatment film to be formed on the surface of the Mg alloymember and the surface of the partner member while the Mg alloy memberand the partner member are connected to each other.

Advantageous Effects of Present Invention

In the metal connection member according to the present disclosure, theMg alloy member and the partner member can be covered with the chemicalconversion treatment film having good adhesion and good corrosionresistance.

The method for a chemical conversion treatment of a metal connectionmember according to the present disclosure enables the chemicalconversion treatment film to be formed on the surface of the Mg alloymember and the surface of the partner member while the Mg alloy memberand the partner member are connected to each other.

Description of Embodiment of Present Invention

First, the aspects of the present invention are listed below.

(1) A metal connection member according to an aspect of the presentinvention includes:

a Mg alloy member composed principally of magnesium;

a partner member connected to the Mg alloy member, the partner memberbeing composed principally of a metal nobler than magnesium; and

a chemical conversion treatment film that covers a surface of the Mgalloy member and a surface of the partner member with covering aboundary between the Mg alloy member and the partner member.

In the above described metal connection member, the Mg alloy member andthe partner member can be covered with the chemical conversion treatmentfilm having good adhesion and good corrosion resistance.

(2) In the above described metal connection member, the charge transferresistance of the Mg alloy member in 1M of Na₂SO₄ is 300 Ω or more and1200 Ω or less.

In such a case, the above described chemical conversion treatment filmcan be formed.

(3) In the above described metal connection member, the content ofaluminum in the Mg alloy member is 0.3% by mass or more and 12.0% bymass or less.

In such a case, the chemical conversion treatment film can be formed.

(4) A method for a chemical conversion treatment of a metal connectionmember according to another aspect of the present invention includes:

a preparation step of preparing a metal connection member, the metalconnection member including a Mg alloy member composed principally ofmagnesium and a partner member connected to the Mg alloy member, thepartner member being composed principally of a metal nobler thanmagnesium; and a chemical conversion treatment step of bringing both ofthe Mg alloy member and the partner member included in the metalconnection member into contact with a chemical conversion treatmentliquid in a collective manner to form a chemical conversion treatmentfilm on a surface of the metal connection member,

wherein the electric conductivity y of the chemical conversion treatmentliquid satisfies at least one of the relation formulae (a) and (b)below,

y≤0.0007 x ₁+14.0   (a)

y≤0.054 x ₂+14.2   (b)

where x₁ (Ω) is the charge transfer resistance of the Mg alloy member in1M of Na₂SO₄, x₂ (mass %) is the content of aluminum in the Mg alloymember, and y (mS/cm) is the electric conductivity of the chemicalconversion treatment liquid.

The above described chemical conversion treatment method enables thechemical conversion treatment film to be formed on the surfaces of theMg alloy member and the partner member as a single piece with coveringthe boundary between the two members while the two members are connectedto each other. This is because, when at least one of the relationformulae (a) and (b) above is satisfied, the rate of formation of thechemical conversion treatment film is not excessively high.

(5) In the method for a chemical conversion treatment of a metalconnection member, the charge transfer resistance xi is 300 Ω or moreand 1200 Ω or less.

When the charge transfer resistance xi is 300 Ω or more, an excessivereaction between the chemical conversion treatment liquid and the Mgalloy member can be suppressed and, consequently, the above describedchemical conversion treatment film can be readily formed. When thecharge transfer resistance x₁ is 1200 Ω or less, the chemical conversiontreatment film can be formed without consuming an excessive amount oftime.

(6) In the method for a chemical conversion treatment of a metalconnection member, the content of aluminum x₂ is 0.3% by mass or moreand 12.0% by mass or less.

In such a case, the chemical conversion treatment film can be readilyformed.

(7) In the method for a chemical conversion treatment of a metalconnection member, the electric conductivity y of the chemicalconversion treatment liquid satisfies 0.1 mS/cm≤y.

In such a case, the chemical conversion treatment film can be readilyformed without consuming an excessive amount of time.

(8) In the method for a chemical conversion treatment of a metalconnection member, the content of zinc in the Mg alloy member is 0.5% bymass or more and 6.2% by mass or less.

In such a case, the chemical conversion treatment film can be readilyformed.

(9) In the method for a chemical conversion treatment of a metalconnection member, the pH of the chemical conversion treatment liquid is2.0 or more and 7.0 or less.

When the pH of the chemical conversion treatment liquid is 2.0 or more,an excessive reaction between the chemical conversion treatment liquidand the Mg alloy member can be suppressed and, consequently, thechemical conversion treatment film can be readily formed. When the pH ofthe chemical conversion treatment liquid is 7.0 or less, an excessivereaction between the chemical conversion treatment liquid and thepartner member can be suppressed and, consequently, the degradation ofthe stability of the chemical conversion treatment liquid can besuppressed. This can reduce the likelihood of troubles occurring duringa continuous operation.

(10) In the method for a chemical conversion treatment of a metalconnection member, the pH and the electric conductivity y of thechemical conversion treatment liquid is adjusted using at least one acidor salt selected from nitric acid, sulfuric acid, hydrofluoric acid,hydrofluosilicic acid, bromic acid, manganic acid, permanganic acid,vanadic acid, hydrogen peroxide, an organic acid, and salts of theseacids.

In such a case, the pH and the electric conductivity y (mS/cm) of thechemical conversion treatment liquid can be readily adjusted.

(11) In the method for a chemical conversion treatment of a metalconnection member, the chemical conversion treatment liquid includes ametal element belonging to Group 4 of the periodic table.

In such a case, the chemical conversion treatment film can be readilyformed.

(12) In the method for a chemical conversion treatment of a metalconnection member, the temperature of the chemical conversion treatmentliquid is 5° C. or more and 70° C. or less.

When the chemical conversion treatment temperature is 5° C. or more, theformation of the chemical conversion treatment film can be readilyaccelerated and the chemical conversion treatment film can be formedwithout consuming an excessive amount of time. When the chemicalconversion treatment temperature is 70° C. or less, the temperature isnot excessively high and the components of the chemical conversiontreatment liquid can readily become stabilized. As a result, thechemical conversion treatment film can be readily formed.

(13) In the method for a chemical conversion treatment of a metalconnection member, the Mg alloy member and the partner member iselectrically connected to each other.

In such a case, the chemical conversion treatment film can be formed onthe surfaces of the Mg alloy member and the partner member as a singlepiece with covering the boundary between the two members. Details of themechanisms are described below.

Details of Embodiment of Present Invention

Details of an embodiment of the present invention are described below.The metal connection member and the method for a chemical conversiontreatment of the metal connection member are described below in thisorder.

[Metal Connection Member]

A metal connection member 1 according to the embodiment is describedbelow with reference to FIG. 1. The metal connection member 1 includes amagnesium (Mg) alloy member 2 and a partner member 3 that is composed ofa specific material and connected to the Mg alloy member 2. One of thefeatures of the metal connection member 1 is that the metal connectionmember 1 includes a chemical conversion treatment film 4 that covers thesurface of the Mg alloy member 2 and the surface of the partner member 3with covering the boundary between the Mg alloy member 2 and the partnermember 3.

Details thereof are described below.

[Mg Alloy Member]

The Mg alloy member 2 is composed of a Mg alloy that includes the Mgelement as a principal component. The term “principal component” usedherein refers to one of the elements constituting the Mg alloy member 2the mass proportion of which is the largest. Examples of the Mg alloyinclude Mg alloys having various compositions (balance: Mg andinevitable impurities) containing Mg and additive elements. Typicalexamples of the Mg alloy include a Mg—Al base alloy. Other examplesthereof include a Mg—Zn based alloy, a Mg RE (rare earth element) basedalloy, and an Y containing alloy.

The Mg Al based alloy contains at least Al as an additive element. Thehigher the Al content, the higher the corrosion resistance and thebetter the mechanical properties, such as strength and plasticdeformation resistance. However, if the Al content is excessively high,plastic formability may become degraded. Accordingly, the Al content ispreferably 0.3% by mass or more and 12.0% by mass or less, is furtherpreferably 5.6% by mass or more and 9.5% by mass or less, and isparticularly preferably 8.3% by mass or more and 9.5% by mass or less.Examples of the additive elements other than Al include one or moreelements selected from Zn, Mn, Si, Be, Ca, Sr, Y, Cu, Ag, Sn, Ni, Au,Li, Zr, Ce, and rare earth elements (except Y and Ce). In the case wherethe Mg Al based alloy contains the above elements, the total content ofthe elements is, for example, 0.01% by mass or more and 10% by mass orless and is preferably 0.1% by mass or more and 5% by mass or less.Examples of the impurities include Fe.

Examples of the more specific composition of the Mg—Al based alloyinclude an AZ based alloy (Mg—Al—Zn based alloy, Zn: 0.2% by mass ormore and 1.5% by mass or less), an AM based alloy (Mg—Al—Mn based alloy,Mn: 0.05% by mass or more and 0.5% by mass or less), an AS based alloy(Mg—Al—Si based alloy, Si: 0.3% by mass or more and 4.0% by mass orless), a Mg—Al-RE (rare earth element) based alloy, an AX based alloy(Mg—Al—Ca based alloy, Ca: 0.2% by mass or more and 6.0% by mass orless), an AZX based alloy (Mg—Al—Zn—Ca based alloy, Zn: 0.2% by mass ormore and 1.5% by mass or less, Ca: 0.1% by mass or more and 4.0% by massor less), and an AJ based alloy (Mg—Al—Sr based alloy, Sr: 0.2% by massor more and 7.0% by mass or less) defined by the ASTM standard.

Typical examples thereof include an AZ31 alloy (Al: 2.5% by mass or moreand 3.5% by mass or less, Zn: 0.6% by mass or more and 1.4% by mass orless) and an AZ91 alloy (Al: 8.3% by mass or more and 9.5% by mass orless, Zn: 0.5% by mass or more and 1.5% by mass or less), which are theAZ based alloys. Typical examples thereof also include an AM60 alloy(Al: 5.6% by mass or more and 6.4% by mass or less, Mn: 0.15% by mass ormore and 0.50% by mass or less), which is the AM based alloy. Otherexamples of the AZ based alloy include AZ10, AZ61, AZ63, AZ80, and AZ81.Other examples of the AM based alloy include AM100.

The Mg—Zn based alloy contains at least Zn as an additive element. TheZn content is preferably 0.5% by mass or more and 6.2% by mass or lessand is particularly preferably 1.5% by mass or more and 4.0% by mass orless. The additive elements other than Zn, the content of the additiveelements, and the impurities are the same as the additive elementscontained in the Mg—Al based alloy, the content of the additiveelements, and the impurities contained in the Mg—Al based alloy,respectively, which are described above.

Examples of the more specific composition of the Mg Zn based alloyinclude a ZX based alloy (Mg—Zn Ca based alloy, Zn: 0.5% by mass or moreand 6.2% by mass or less, Ca: 0.05% by mass or more and 0.3% by mass orless) and a ZE based alloy (Mg—Zn-RE based alloy, Zn: 0.5% by mass ormore and 6.2% by mass or less, rare earth elements: 0.05% by mass ormore and 0.5% by mass or less). Other examples thereof include aMg—Zn—Sr based alloy, a Mg—Zn—Ba based alloy, a Mg—Zn—Ca-RE based alloy,a Mg—Zn—RE-Mn based alloy, a Mg—Zn—Sr-RE based alloy, and a Mg—Zn—Ba-REbased alloy. Typical examples thereof include ZX10, which is a ZX basedalloy.

The corrosion resistance of a Mg alloy varies with the compositionthereof. Since the state of formation of the chemical conversiontreatment film 4 and the rate of formation of the chemical conversiontreatment film 4 vary with corrosion resistance, it is preferable tospecify the corrosion resistance of the Mg alloy. On the basis of aTafel distribution curve, a corrosion current density in the standardstate can be used as an index. The charge transfer resistance of the Mgalloy measured using 1M of Na₂SO₄ as a solvent and a silver electrode asa counter electrode is preferably 300 Ω or more and 1200 Ω or less. Theunit “M” used herein refers to volume molar concentration: mol/L (dm³).When the charge transfer resistance is 300 Ω or more, an excessivereaction between the chemical conversion treatment liquid and the Mgalloy member 2 can be suppressed and, consequently, the chemicalconversion treatment film 4l can be readily formed on the surfaces ofthe members 2 and 3 as a single piece with covering the boundary betweenthe members 2 and 3. When the charge transfer resistance is 1200 Ω orless, the above described chemical conversion treatment film 4 can beformed without consuming an excessive amount of time. The chargetransfer resistance is further preferably 400 Ω or more and 1100 Ω orless and is particularly preferably 800 Ω or more and 1050 Ω or less.

Examples of the type of the Mg alloy member 2 include a cast materialprepared by casting, such as twin roll or die casting; a rolled materialprepared by rolling the cast material; a processed material prepared bysubjecting the rolled material to a heat treatment, a leveling process,polishing processing, or the like; and a plastic formed materialprepared by subjecting the rolled material or the processed material toplastic forming. The Mg alloy member 2 can be subjected to a solutiontreatment prior to the rolling. The shape of the Mg alloy member 2 maybe selected appropriately. Typical examples thereof include a plate likeshape. Although the size of the Mg alloy member 2 can be selectedappropriately, it is preferable that the surface area ratio describedbelow be satisfied.

[Partner Member]

The partner member 3 is formed of a material composed principally of ametal nobler than the Mg alloy. The metal nobler than the Mg alloy is ametal having a lower ionization tendency than the Mg alloy. Specificexamples of the partner member 3 include an Fe member composed of an Febased material and an Al member composed of an Al based material. Theterm “Fe based material” used herein refers to pure Fe or an Fe alloycomposed principally of the Fe element. The meaning of the term“principally” is the same as for the Mg alloy described above. Examplesof the iron alloy include a steel, a stainless steel alloy, and a carbonsteel. The partner member 3 is, for example, a steel member composed ofa steel. The term “Al based material” used herein refers to pure Al oran Al alloy composed principally of the Al element. The meaning of theterm “principally” is the same as for the Mg alloy described above.Examples of the Al alloy include an Al—Mg based alloy (5000 seriesalloy) and an Al—Mg—Si based alloy (6000 series alloy).

The number of the partner members 3 may be one or two or more. In thecase where the number of the partner members 3 is two or more, at leastone of the partner members 3 may be one of the Fe member and the Almember, and the other partner members 3 may be members formed of amaterial composed principally of a metal that is nobler than the Mgelement and is other than the principal element (Fe or Al) of the atleast one partner member 3. For example, the other partner members 3 maybe Zn members composed of a Zn based material. That is, the partnermembers 3 may be the Fe member and the Al member only or may be at leastone of the Fe member and the Al member and the Zn member.

The shape of the partner member 3 can be selected appropriately. Typicalexamples thereof include a plate like shape, as for the Mg alloy member2. Although the size of the partner member 3 may be selectedappropriately, it is preferable that the surface area ratio describedbelow be satisfied.

(Surface Area Ratio)

Although the surface area ratio between the surface area of the Mg alloymember 2 and the surface area of the partner member 3 (Surface area ofMg alloy member 2 Entire surface area) may be selected appropriately,the surface area ratio is preferably 0.1% or more and 50% or less, isfurther preferably 0.1% or more and 10% or less, and is particularlypreferably 0.1% or more and 3% or less. The current that flows in the Mgalloy member 2 and the partner member 3 varies with the above surfacearea ratio. When the surface area ratio is 0.1% or more, a homogeneouschemical conversion treatment film 4 can be readily formed on thesurfaces of the members 2 and 3 as a single piece with covering theboundary between the members 2 and 3. When the surface area ratio is 50%or less, the amount of time required by the chemical conversiontreatment can be readily reduced. The entire surface area used forcalculating the surface area ratio does not include the contact surfaceat which the Mg alloy member 2 and the partner member 3 come intocontact with each other.

The mode of connection between the Mg alloy member 2 and the partnermember 3 may be selected appropriately in accordance with, for example,the type of the material constituting the partner member 3. Theconnection between the Mg alloy member 2 and the partner member 3 can beestablished by, for example, clamping with bolts and nuts, clampingusing rivets, welding, or friction stir welding. In the case where boltsare used, for example, through holes into which the bolts are to beinserted are formed in the members 2 and 3. In the case where rivets areused, for example, rivets are formed in one of the members 2 and 3 andholes into which the rivets are to be inserted are formed in the othermember.

[Chemical Conversion Treatment Film]

The chemical conversion treatment film 4 covers the surface of the Mgalloy member 2 and the surface of the partner member 3 with covering theboundary therebetween. That is, the chemical conversion treatment film 4is arranged to cover the boundary between the Mg alloy member 2 and thepartner member 3 so as to extend from one of the members 2 and 3 to theother member. The region covered with the chemical conversion treatmentfilm 4 varies with the mode of connection between the Mg alloy member 2and the partner member 3 and is, for example, a region other than thecontact surface at which the Mg alloy member 2 and the partner member 3come into contact with each other.

The chemical conversion treatment film 4 includes an oxide of an elementbelonging to Group 4 of the periodic table as a principal component. TheGroup 4 element is zirconium (Zr), titanium (Ti), or hafnium (Hf). Theoxide thereof varies with the components of the chemical conversiontreatment liquid and may be, for example, zirconium oxide (ZrO₂),zirconium nitrate (Zr(NO₃)₄), or zirconium fluorophosphate (ZrFPO₄). Thechemical conversion treatment film 4 may further include a fluoride ofthe Group 4 element (e.g., ZrF₄), a fluoride that does not include theGroup 4 element (e.g., HF, NH₄HF₂, NH₄F, NaHF₂, or NaF), or the like. Achemical conversion treatment film 4 formed on the surface of the Mgalloy member 2 may further include an oxide or hydroxide of Mg. Achemical conversion treatment film 4 formed on the surface of thepartner member 3 may further include an oxide or hydroxide of theprincipal element (Fe or Al) of the partner member 3, a fluoride of theprincipal element, or a compound containing the principal element, theoxygen element, and the fluorine element. The materials constituting thechemical conversion treatment film 4 and the contents of the materialsmay be determined by X ray fluorescence analysis (XRF).

The thickness of a region of the chemical conversion treatment film 4which covers the Mg alloy member 2 and the thickness of a region of thechemical conversion treatment film 4 which covers the partner member 3are preferably, for example, 10 nm or more and 300 nm or less. When thethicknesses of these regions are 10 nm or more, corrosion resistance canbe readily enhanced. When the thicknesses of these regions are 300 nm orless, the thicknesses of these regions are not excessively large.

The thicknesses of the above regions are further preferably 20 nm ormore and 250 nm or less and are particularly preferably 50 nm or moreand 200 nm or less. The thickness of a region of the chemical conversiontreatment film 4 which covers the Mg alloy member 2 and the thickness ofa region of the chemical conversion treatment film 4 which covers thepartner member 3 may be equal to or different from each other. Thethickness of the above regions can be measured by observing a crosssection with a scanning electron microscope (SEM). Specifically, thethickness of the chemical conversion treatment film 4 is measured atplural (e.g., five or more) positions in a cross section of each region,and the average thereof is considered the thickness of the region of thechemical conversion treatment film 4 which covers the member.

[Others]

The metal connection member 1 can further include a coating film (notillustrated in the drawings) that covers the surface of the chemicalconversion treatment film 4. The structure of the coating film may be asingle layer structure or a multilayer structure. Examples of thematerial constituting the coating film include an acrylic resin.Examples of the coating material constituting the coating film includeMG Net T (MG Net is a registered trademark) produced by Origin Co., Ltd.and RYLCON BB20 (RYLCON is a registered trademark) and ARMOR TOP AT20(product name) produced by Musashi Paint Holdings Co., Ltd. The coatingfilm may have, for example, a single layer structure consisting of theabove coating material or a multilayer structure that includes a lowerlayer that is disposed immediately on the chemical conversion treatmentfilm 4 and composed of an electrodeposition coating material and one ormore upper layers that are disposed immediately on the lower layer andcomposed of the above coating material.

[Applications]

The metal connection member 1 according to the embodiment can besuitably used as a part of automobiles.

[Actions and Effects]

In the metal connection member 1 according to the embodiment, the Mgalloy member 2 and the partner member 3 can be covered with the chemicalconversion treatment film 4 as a single piece.

[Method for Chemical Conversion Treatment of Metal Connection Member]

A method for a chemical conversion treatment of the metal connectionmember according to the embodiment is described below. The method for achemical conversion treatment of the metal connection member accordingto the embodiment includes a preparation step of preparing a metalconnection member that includes a magnesium (Mg) alloy member 2 and apartner member 3 that is connected to the Mg alloy member 2 and composedof a specific material; and a chemical conversion treatment step ofsubjecting the metal connection member to a chemical conversiontreatment. One of the features of the method for a chemical conversiontreatment of the metal connection member is that the Mg alloy member 2and the partner member 3 included in the metal connection member arebrought into contact with a specific chemical conversion treatmentliquid in a collective manner. Details of the above steps are describedbelow.

[Preparation Step]

In the preparation step, a metal connection member that includes theabove described Mg alloy member 2 and the above described partner member3 that are connected to each other, which is the member that is to besubjected to a chemical conversion treatment, is prepared.

[Chemical Conversion Treatment Step]

In the chemical conversion treatment step, the metal connection memberis subjected to a chemical conversion treatment in order to form achemical conversion treatment film 4 on the surfaces of the Mg alloymember 2 and the partner member 3 included in the metal connectionmember. In the chemical conversion treatment, the Mg alloy member 2 andthe partner member 3 included in the metal connection member are broughtinto contact with a common chemical conversion treatment liquid in acollective manner. Specifically, for example, an immersion method inwhich the metal connection member is immersed in the chemical conversiontreatment liquid and a coating method in which the chemical conversiontreatment liquid is applied to the metal connection member by sprayingor the like can be suitably used. In the coating method, the chemicalconversion treatment liquid is applied onto the surfaces of the Mg alloymember 2 and the partner member 3 so as to form a coating film thatcovers the boundary between the members 2 and 3 as a single piece.

Upon the metal connection member being brought into contact with thechemical conversion treatment liquid, a current flows from the chemicalconversion treatment liquid to each of the Mg alloy member 2 and thepartner member 3. Since the partner member 3 is composed of a metalnobler than that constituting the Mg alloy member 2, a galvanic currentflows from the partner member 3 to the Mg alloy member 2. In the casewhere the metals are not electrically connected to each other, thecurrent flows through the coating liquid. The galvanic current thatoccurs in the early stage of the contact between the metal connectionmember and the chemical conversion treatment liquid is larger than thecurrent that flows from the chemical conversion treatment liquid to thepartner member 3. While this relationship is satisfied, the chemicalconversion treatment film 4 is more likely to be formed on the surfaceof the Mg alloy member 2 than on the surface of the partner member 3.This is because the galvanic current accelerates the formation of thechemical conversion treatment film 4 on the Mg alloy member 2. When theformation of the chemical conversion treatment film 4 on the surface ofthe Mg alloy member 2 starts, the electric resistance of the Mg alloymember 2 increases and the current that flows from the chemicalconversion treatment liquid to the partner member 3 becomes larger thanthe galvanic current. In this stage, the chemical conversion treatmentfilm 4 is formed on the surface of the partner member 3. As a result,the chemical conversion treatment film 4 can be formed, as a singlepiece, on the surfaces of the Mg alloy member 2 and the partner member 3that are connected to each other, with covering the boundary between themembers 2 and 3.

The chemical conversion treatment liquid is a coating liquid used forforming the chemical conversion treatment film 4 on the surfaces of theMg alloy member 2 and the partner member 3 of the metal connectionmember. The chemical conversion treatment liquid preferably contains anelement belonging to Group 4 of the periodic table.

In such a case, a chemical conversion treatment film 4 including anoxide of the Group 4 element can be formed on the surfaces of the Mgalloy member 2 and the partner member 3 of the metal connection member.The Group 4 element may be included in the chemical conversion treatmentliquid, for example, in the form of a fluoride. For example, examples ofa fluoride of Zr include H₂ZrF₆ (fluorozirconic acid) and (NH₄)₂ZrF₆(ammonium hexafluorozirconate), which is an ammonium salt offluorozirconic acid.

The chemical conversion treatment liquid may further contain one or moresubstances selected from nitric acid, a nitrate, an organic acid, anorganic acid salt, boric acid (e.g., HBF₄ (tetrafluoroboric acid)), aborate, phosphoric acid, a phosphate, sulfuric acid, a sulfate,hydrofluoric acid, a hydrofluoride, hydrofluosilicic acid, ahydrofluosilicate, bromic acid, a bromate, manganic acid, a manganate,permanganic acid, a permanganate, vanadic acid, a vanadate, hydrogenperoxide, and a hydrogen peroxide salt. The above acids and salts adjustthe electric conductivity y and pH of the chemical conversion treatmentliquid which are described below. Specifically, the higher the contentsof the above acids and salts, the higher the electric conductivity y andpH of the chemical conversion treatment liquid. Among these, thephosphoric acid and the phosphate are preferably used in amounts atwhich they do not affect the main reaction of the chemical conversiontreatment (are not formed as the chemical conversion treatment film 4).The chemical conversion treatment liquid may further contain vanadium orsodium gluconate. Specifically, the chemical conversion treatment liquidmay contain, for example, as a principal component, a substance preparedby adding the above described fluoride of Zr and hydrofluoric acid towater used as a solvent. In the present disclosure, pH refers tohydrogen ion exponent.

The chemical conversion treatment liquid may contain ions of one or moreelements selected from Mg, Al, Si, P, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn,Sr, Sn, Y, La, rare earth elements (except Y and La), and the like. Thetotal content thereof may be, for example, by mass, more than 0 ppm and10000 ppm or less, is further preferably 5000 ppm or less, and isparticularly preferably 200 ppm or less. The above ions substantially donot change the electric conductivity y (mS/cm), pH, and treatmenttemperature of the chemical conversion treatment liquid.

The electric conductivity y (mS/cm) of the chemical conversion treatmentliquid may be selected appropriately in accordance with the compositionof the Mg alloy member 2 and the material constituting the joint member.At least one of the relation formulae (a) and (b) below is satisfied,where x₁ (Ω) is the charge transfer resistance of the Mg alloy member 2,and x₂ (mass %) is the aluminum content in the Mg alloy member 2. Notethat, the electric conductivity y of the chemical conversion treatmentliquid is an approximate value obtained by rounding the calculated valueto the first decimal place. When at least one of the relation formulae(a) and (b) below is satisfied, the chemical conversion treatment film 4can be formed on the surfaces of the Mg alloy member 2 composedprincipally of Mg, which is a metal more base than Al relative to Fe,and the partner member 3 as a single piece with covering the boundarybetween the members 2 and 3 while the members 2 and 3 are connected toeach other. This is because the rate of precipitation of the componentsof the chemical conversion treatment film 4 is not excessively high(e.g., 1.2 mg/m²·s or less) and, accordingly, the rate of formation ofthe chemical conversion treatment film 4 is not excessively high. It isneedless to say that both formulae (a) and (b) below are preferablysatisfied.

y≤0.0007 x ₁+14.0   (a)

y≤0.054 x ₂+14.2   (b)

The electric conductivity y of the chemical conversion treatment liquidpreferably satisfies 0.1 mS/cm≤y. When the electric conductivity y is0.1 mS/cm or more, the certain pH and the certain ion concentrationadequate for forming the chemical conversion treatment film 4 can bemaintained. This enables the above described chemical conversiontreatment film to be readily formed without consuming an excessiveamount of time. Although the chemical conversion treatment film 4 can beformed by performing the chemical conversion treatment for a long periodof time even in the case where the electric conductivity y satisfiesy<0.1 mS/cm (e.g., y=about 0.01 mS/cm), it is preferable to satisfy 0.1mS/cm≤y as described above in consideration of a realistic chemicalconversion treatment time.

For example, the electric conductivity y of the chemical conversiontreatment liquid is preferably 0.2 mS/cm or more and 12 mS/cm or less,is further preferably 7.0 mS/cm or less, and is particularly preferably5.0 mS/cm or more and 6.0 mS/cm or less.

The pH of the chemical conversion treatment liquid is preferably 2.0 ormore and 7.0 or less. When the pH of the chemical conversion treatmentliquid is 2.0 or more, an excessive reaction between the chemicalconversion treatment liquid and the Mg alloy member 2 can be suppressedand, consequently, the chemical conversion treatment film 4 can bereadily formed. When the pH of the chemical conversion treatment liquidis 7.0 or less, an excessive reaction between the chemical conversiontreatment liquid and the partner member 3 can be suppressed and,consequently, the degradation of the stability of the chemicalconversion treatment liquid can be suppressed. This can reduce thelikelihood of troubles occurring during a continuous operation. The pHof the chemical conversion treatment liquid is further preferably 2.0 ormore and 6.0 or less and is particularly preferably 2.5 or more and 4.5or less.

The temperature of the chemical conversion treatment liquid can beselected appropriately in accordance with the material constituting thepartner member 3, the type of the chemical conversion treatment liquid,and the like and is, for example, 5° C. or more and 70° C. or less. Whenthe temperature of the chemical conversion treatment liquid is 5° C. ormore, the formation of the chemical conversion treatment film 4 can bereadily accelerated and, consequently, the chemical conversion treatmentfilm 4 can be formed without consuming an excessive amount of time. Whenthe temperature of the chemical conversion treatment liquid is 70° C. orless, the temperature is not excessively high and the components of thechemical conversion treatment liquid can readily become stabilized. As aresult, a homogeneous chemical conversion treatment film 4 can bereadily formed. The temperature of the chemical conversion treatmentliquid may be, for example, 10° C. or more and 60° C. or less and, inparticular, 30° C. or more and 60° C. or less.

The amount of time during which the chemical conversion treatment isperformed is not limited and is preferably 30 minutes or less. In such acase, the amount of treatment time is not excessively large. The abovechemical conversion treatment time is preferably 1 minute or more. Insuch a case, a homogeneous chemical conversion treatment film 4 can bereadily formed. The chemical conversion treatment time is furtherpreferably 1 minute or more and 3 minutes or less and is particularlypreferably about 2 minutes.

[Other Steps]

The method for a chemical conversion treatment of the metal connectionmember can further include a coating step. In the coating step, acoating film is formed on the surface of the chemical conversiontreatment film 4. The formation of the coating film can be done byelectrodeposition coating or the like.

[Applications]

The method for a chemical conversion treatment of the metal connectionmember according to the embodiment can be used as a chemical conversiontreatment method in which a chemical conversion treatment film is formedon the surfaces of the Mg alloy member and the partner member connectedto each other.

[Actions and Effects]

The method for a chemical conversion treatment of the metal connectionmember according to the embodiment enables the chemical conversiontreatment film 4 to be formed on the surfaces of the Mg alloy member 2and the partner member 3 as a single piece with covering the boundarybetween the members 2 and 3 while the members 2 and 3 are connected toeach other.

TEST EXAMPLE 1

Metal connection members including the Mg alloy member and the partnermember connected to each other were subjected to a chemical conversiontreatment and a coating treatment. The state of formation of a chemicalconversion treatment film on the surfaces of the two members wasevaluated.

[Preparation Step]

(Sample Nos. 1-1 to 1-34)

Each of the metal connection members prepared in Sample Nos. 1-1 to 1-34included a rectangular sheet composed of a specific one of the alloysdescribed in Table 1, which served as the Mg alloy member, and arectangular SPCC (cold rolled steel sheet) conforming to “Cold reducedcarbon steel sheet and strip, JIS G 3141 (2017)”, which served as thepartner member, the two members being connected to each other. The sizeof the Mg alloy member and the partner member used for measuring thegalvanic current was 10 mm×40 mm×1.0 mm thick. The Mg alloy member andthe partner member were connected to a zero shunt ammeter (HM 103Aproduced by HOKUTO DENKO CORPORATION) with a copper wire in order tosimulate a state electrically equivalent to the state in which the Mgalloy member and the partner member are directly connected to eachother. The size of the Mg alloy member and the partner member used forevaluating the state of formation of the chemical conversion treatmentfilm was 30 mm×100 mm×1.0 mm thick. The two members were connected toeach other by superimposing the members on each other such that themembers partially overlap each other and then clamping the members withbolts and nuts. The holes into which the bolts were inserted were formedin the portions of the two members which overlap each other. The size ofthe Mg alloy member and the partner member was set such that the surfacearea ratio of Mg alloy member Entirety was a corresponding one of thevalues described in Tables 2 to 7. The metal connection members ofSample Nos. 1-1 to 1-34 were not pickled prior to the chemicalconversion treatment.

(Sample Nos. 2-1 to 2-18)

Each of the metal connection members prepared in Sample Nos. 2-1 to 2-18included a rectangular sheet composed of a specific one of the alloysdescribed in Table 1, which served as the Mg alloy member, and arectangular 5000 series Al alloy A5052, which served as the partnermember, the two members being connected to each other. The size of theMg alloy member and the partner member used for measuring the galvaniccurrent was the same as in Sample No. 1-1, etc. The size of the Mg alloymember and the partner member used for evaluating the state of formationof the chemical conversion treatment film was 60 mm×100 mm×1.0 mm thick.The two members were connected to each other in the same manner as inSample No. 1-1, etc. The size of the Mg alloy member and the partnermember was set such that the surface area ratio of Mg alloy memberEntirety was a corresponding one of the values described in Table 8. Themetal connection members of Sample Nos. 2-1 to 1-18 were not pickledprior to the chemical conversion treatment.

[Chemical Conversion Treatment Step]

The metal connection members were subjected to a chemical conversiontreatment. In the chemical conversion treatment, three types of chemicalconversion treatment liquids A to C were used. The chemical conversiontreatment liquids A to C were prepared by adding nitric acid and ammoniato a chemical conversion treatment liquid containing Zr (Grander AL80produced by MILLION CHEMICALS CO., LTD.). The chemical conversiontreatment liquids A to C contain, in addition to a Mg ion, Fe and Alions in different combinations as described below. The contents of theFe and Al ions were adjusted to be trace such that electric conductivityand pH were not changed. Specifically, the contents of the Fe and Alions were adjusted to be about 100 ppm by mass. The above adjustment wasmade by immersing the SPCC or the A5052 in the chemical conversiontreatment liquids before the samples were immersed in the chemicalconversion treatment liquids. The electric conductivity and pH of eachof the chemical conversion treatment liquids were set to the valuesdescribed in Tables 2 to 8 by adjusting the amounts of nitric acid andammonia added. Electric conductivity and pH were determined using acommercial multi function water quality meter (MM 60R produced by DKKTOA CORPORATION).

Chemical conversion treatment liquid A: contains Fe ion (102 ppm) and Mgion

Chemical conversion treatment liquid B: contains Al ion (108 ppm) and Mgion

Chemical conversion treatment liquid C: contains Fe ion (107 ppm), Alion (99 ppm), and Mg ion

The Mg alloy member and the partner member connected to each other wereimmersed in a specific one of the chemical conversion treatment liquidsA to C in a collective manner. The temperature and treatment time of thechemical conversion treatment liquid were set as described in Tables 2to 8. Subsequently, the amount of Zr precipitated (mg/m²) and thegalvanic current (mA) were measured. Tables 2 to 8 summarize theresults. Tables 2 and 3 summarize the results obtained when the metalconnection members of Sample Nos. 1-1 to 1-34 were immersed in thechemical conversion treatment liquid A. Tables 4 and 5 summarize theresults obtained when the metal connection members of Sample Nos. 1-1 to1-34 were immersed in the chemical conversion treatment liquid B. Tables6 and 7 summarize the results obtained when the metal connection membersof Sample Nos. 1-1 to 1-34 were immersed in the chemical conversiontreatment liquid C. Table 8 summarizes the results obtained when themetal connection members of Sample Nos. 2-1 to 2-18 were immersed in thechemical conversion treatment liquid C. Galvanic current was measuredusing the above zero shunt ammeter. The amount of Zr precipitated wasdetermined from the galvanic current on the basis of the Faraday'ssecond law.

[Coating Step]

The chemical conversion treatment film of each of the samples wasovercoated with ARMOR TOP AT20 produced by Musashi Paint Holdings Co.,Ltd.

[Evaluation of Appearance]

The appearance of each of the samples was visually inspected. Theappearance of the sample was evaluated as Good when the surface of thesample was smooth. The appearance of the sample was evaluated as Badwhen irregularities or air bubbles were confirmed. Tables 2 to 8summarize the results.

[Evaluation of Corrosion Resistance]

The corrosion resistance of each of the samples was evaluated. Thisevaluation was conducted conforming to “Methods of salt spray testing,JIS Z 2371(2000)” and “JIS K5600 5 6:1999 (Testing methods for paintsMechanical property of film Adhesion test (Cross cut test)”.Specifically, for each of the samples, the coating films and thechemical conversion treatment films formed on the Mg alloy member andthe partner member were cross cut, and the resulting samples weresubjected to a salt spray test under the following test conditions. Inthe salt spray test, a salt spray test instrument (STP 90V produced bySuga Test Instruments Co., Ltd.) was used. After the test, the swellingand peeling of the coating film was observed on the surface of the Mgalloy member and on the surface of the partner member. Corrosionresistance was evaluated as Good when the maximum width of swelling orpeeling of the coating film observed on the surfaces of the Mg alloymember and the partner member was 2.0 mm or less. Corrosion resistancewas evaluated as Bad when the above maximum width was more than 2.0 mm.Tables 2 to 8 summarize the results.

[Salt Spray Test Conditions]

Saltwater concentration: 5%

Testing temperature: 35° C.

Testing time: 960 h

[Evaluation of Adhesion]

The secondary adhesion of each of the samples was evaluated after thesalt spray test. This evaluation was conducted by a cross cut test. Inthis test, notches that reached the Mg alloy member or the partnermember were formed in the coating films and the chemical conversiontreatment films disposed on the Mg alloy member and the partner memberat intervals of 1 mm in the vertical and horizontal directions to createa 10×10 grid pattern. An adhesive tape was stuck onto the grid patternand then removed in order to evaluate the adhesion of the coating film.Whether or not detachment (peeling) occurred in the cells of the gridwas visually inspected and the number of cells in which peeling occurredwas counted. The adhesion of the coating film was evaluated as Good whendetachment did not occur in any of the cells (100 cells+100 cells). Theadhesion of the coating film was evaluated as Bad when detachmentoccurred even in only one cell. Tables 2 to 8 summarize the results.

TABLE 1 Charge transfer Amount of Alloy resistance x₁ Al content x₂ Znadded type [Ω] [mass %] [mass %] AZ91 1050 9 0.7 AZX911 1000 8.9 0.7AM60 800 6 — AZ31 560 3 0.8 ZX10 350 0 1.5

TABLE 2 Electric Surface Treatment Coating film Galvanic Amount of ZrSample Alloy conductivity area ratio time Temperature Corrosion currentprecipitated No. type [mS/cm] [%] [minute] [° C.] resistance Adhesion[mA] pH [mg/m²] Appearance 1-1 AZ91 18 3 2 30 Good Bad 1047.33 2.1401.27 Bad 1-2 AZ91 14.7 3 2 30 Good Good 808.48 2.1 309.76 Good 1-3AZ91 12 3 2 30 Good Good 613.68 2.1 235.12 Good 1-4 AZ91 7 3 2 30 GoodGood 250.82 2.2 96.10 Good 1-5 AZ91 6 3 2 30 Good Good 178.50 2.9 68.39Good 1-6 AZ91 5 3 2 30 Good Good 106.18 3.8 40.68 Good 1-7 AZ91 0.2 3 230 Good Good 0.59 4.5 0.22 Good 1-8 AZ91 0.1 3 30 30 Good Good 0.19 5.11.07 Good 1-9 AZ91 0.05 3 30 30 Bad Bad 0.01 5.1 0.06 Bad 1-10 AZ91 0.053 30 4 Bad Bad 0.07 2.1 0.41 Bad 1-11 AZ91 0.1 3 30 5 Good Good 0.10 2.10.55 Good 1-12 AZ91 6 3 2 10 Good Good 83.27 2.9 31.90 Good 1-13 AZ91 63 2 60 Good Good 559.19 2.9 214.24 Good 1-14 AZ91 14.7 3 1 70 Good Good969.26 5.1 185.68 Good 1-15 AZ91 15 3 1 73 Good Bad 1016.93 5.1 194.81Bad 1-16 AZ91 6 10 2 30 Good Good 168.83 2.9 64.68 Good 1-17 AZ91 0.1 5030 30 Good Good 0.09 2.1 0.54 Good 1-18 AZ91 0.05 60 30 30 Bad Bad 0.052.1 0.29 Bad

TABLE 3 Electric Surface Treatment Coating film Galvanic Amount of ZrSample Alloy conductivity area ratio time Temperature Corrosion currentprecipitated No. type [mS/cm] [%] [minute] [° C.] resistance Adhesion[mA] pH [mg/m²] Appearance 1-19 AZX911 0.1 3 30 30 Good Good 0.17 5.10.98 Good 1-20 AZX911 6 3 2 30 Good Good 175.19 2.9 67.12 Good 1-21AZX911 14.7 3 2 30 Good Good 819.94 2.1 314.15 Good 1-22 AZX911 15 3 130 Good Bad 1033.83 2.1 198.05 Bad 1-23 AM60 0.1 3 2 30 Good Good 0.515.1 0.20 Good 1-24 AM60 6 3 2 30 Good Good 192.00 2.9 73.56 Good 1-25AM60 14.5 3 2 30 Good Good 842.86 2.1 322.93 Good 1-26 AM60 15 3 1 30Good Bad 1089.35 2.1 208.68 Bad 1-27 AZ31 0.1 3 2 30 Good Good 0.53 5.10.20 Good 1-28 AZ31 6 3 2 30 Good Good 205.49 2.9 78.73 Good 1-29 AZ3114.4 3 2 30 Good Good 858.13 2.1 328.78 Good 1-30 AZ31 15 3 1 30 GoodBad 1176.43 2.1 225.37 Bad 1-31 ZX10 0.1 3 2 30 Good Good 0.53 5.1 0.20Good 1-32 ZX10 6 3 2 30 Good Good 219.24 2.9 84.00 Good 1-33 ZX10 14.2 32 30 Good Good 870.87 2.1 333.66 Good 1-34 ZX10 15 3 1 30 Good Bad1249.26 2.1 239.32 Bad

TABLE 4 Electric Surface Treatment Coating film Galvanic Amount of ZrSample Alloy conductivity area ratio time Temperature Corrosion currentprecipitated No. type [mS/cm] [%] [minute] [° C.] resistance Adhesion[mA] pH [mg/m²] Appearance 1-1 AZ91 18 3 2 30 Good Bad 1001.41 2.1383.68 Bad 1-2 AZ91 14.7 3 2 30 Good Good 773.03 2.1 296.18 Good 1-3AZ91 12 3 2 30 Good Good 586.77 2.1 224.81 Good 1-4 AZ91 7 3 2 30 GoodGood 239.82 2.2 91.88 Good 1-5 AZ91 6 3 2 30 Good Good 170.68 2.9 65.39Good 1-6 AZ91 5 3 2 30 Good Good 101.53 3.8 38.90 Good 1-7 AZ91 0.2 3 230 Good Good 0.56 4.5 0.21 Good 1-8 AZ91 0.1 3 30 30 Good Good 0.18 5.11.03 Good 1-9 AZ91 0.05 3 30 30 Bad Bad 0.01 5.1 0.06 Bad 1-10 AZ91 0.053 30 4 Bad Bad 0.07 2.1 0.39 Bad 1-11 AZ91 0.1 3 30 5 Good Good 0.09 2.10.52 Good 1-12 AZ91 6 3 2 10 Good Good 79.62 2.9 30.50 Good 1-13 AZ91 63 2 60 Good Good 534.67 2.9 204.85 Good 1-14 AZ91 14.7 3 1 70 Good Good926.76 5.1 177.54 Good 1-15 AZ91 15 3 1 73 Good Bad 972.34 5.1 186.27Bad 1-16 AZ91 6 10 2 30 Good Good 161.42 2.9 61.85 Good 1-17 AZ91 0.1 5030 30 Good Good 0.09 2.1 0.51 Good 1-18 AZ91 0.05 60 30 30 Bad Bad 0.052.1 0.28 Bad

TABLE 5 Electric Surface Treatment Coating film Galvanic Amount of ZrSample Alloy conductivity area ratio time Temperature Corrosion currentprecipitated No. type [mS/cm] [%] [minute] [° C.] resistance Adhesion[mA] pH [mg/m²] Appearance 1-19 AZX911 0.1 3 30 30 Good Good 0.16 5.10.93 Good 1-20 AZX911 6 3 2 30 Good Good 167.51 2.9 64.18 Good 1-21AZX911 14.7 3 2 30 Good Good 783.99 2.1 300.37 Good 1-22 AZX911 15 3 130 Good Bad 988.51 2.1 189.37 Bad 1-23 AM60 0.1 3 2 30 Good Good 0.495.1 0.19 Good 1-24 AM60 6 3 2 30 Good Good 183.58 2.9 70.34 Good 1-25AM60 14.5 3 2 30 Good Good 805.90 2.1 308.77 Good 1-26 AM60 15 3 1 30Good Bad 1041.59 2.1 199.53 Bad 1-27 AZ31 0.1 3 2 30 Good Good 0.51 5.10.20 Good 1-28 AZ31 6 3 2 30 Good Good 196.48 2.9 75.28 Good 1-29 AZ3114.4 3 2 30 Good Good 820.51 2.1 314.37 Good 1-30 AZ31 15 3 1 30 GoodBad 1124.85 2.1 215.49 Bad 1-31 ZX10 0.1 3 2 30 Good Good 0.51 5.1 0.20Good 1-32 ZX10 6 3 2 30 Good Good 209.63 2.9 80.32 Good 1-33 ZX10 14.2 32 30 Good Good 832.68 2.1 319.03 Good 1-34 ZX10 15 3 1 30 Good Bad1194.49 2.1 228.82 Bad

TABLE 6 Electric Surface Treatment Coating film Galvanic Amount of ZrSample Alloy conductivity area ratio time Temperature Corrosion currentprecipitated No. type [mS/cm] [%] [minute] [° C.] resistance Adhesion[mA] pH [mg/m²] Appearance 1-1 AZ91 18 3 2 30 Good Bad 1022.39 2.1391.71 Bad 1-2 AZ91 14.7 3 2 30 Good Good 789.23 2.1 302.38 Good 1-3AZ91 12 3 2 30 Good Good 599.07 2.1 229.52 Good 1-4 AZ91 7 3 2 30 GoodGood 244.85 2.2 93.81 Good 1-5 AZ91 6 3 2 30 Good Good 174.25 2.9 66.76Good 1-6 AZ91 5 3 2 30 Good Good 103.66 3.8 39.71 Good 1-7 AZ91 0.2 3 230 Good Good 0.57 4.5 0.22 Good 1-8 AZ91 0.1 3 30 30 Good Good 0.18 5.11.05 Good 1-9 AZ91 0.05 3 30 30 Bad Bad 0.01 5.1 0.06 Bad 1-10 AZ91 0.053 30 4 Bad Bad 0.07 2.1 0.40 Bad 1-11 AZ91 0.1 3 30 5 Good Good 0.09 2.10.53 Good 1-12 AZ91 6 3 2 10 Good Good 81.28 2.9 31.14 Good 1-13 AZ91 63 2 60 Good Good 545.87 2.9 209.14 Good 1-14 AZ91 14.7 3 1 70 Good Good946.18 5.1 181.26 Good 1-15 AZ91 15 3 1 73 Good Bad 992.71 5.1 190.17Bad 1-16 AZ91 6 10 2 30 Good Good 164.81 2.9 63.14 Good 1-17 AZ91 0.1 5030 30 Good Good 0.09 2.1 0.52 Good 1-18 AZ91 0.05 60 30 30 Bad Bad 0.052.1 0.29 Bad

TABLE 7 Electric Surface Treatment Coating film Galvanic Amount of ZrSample Alloy conductivity area ratio time Temperature Corrosion currentprecipitated No. type [mS/cm] [%] [minute] [° C.] resistance Adhesion[mA] pH [mg/m²] Appearance 1-19 AZX911 0.1 3 30 30 Good Good 0.17 5.10.95 Good 1-20 AZX911 6 3 2 30 Good Good 171.02 2.9 65.52 Good 1-21AZX911 14.7 3 2 30 Good Good 800.42 2.1 306.67 Good 1-22 AZX911 15 3 130 Good Bad 1009.22 2.1 193.33 Bad 1-23 AM60 0.1 3 2 30 Good Good 0.505.1 0.19 Good 1-24 AM60 6 3 2 30 Good Good 187.43 2.9 71.81 Good 1-25AM60 14.5 3 2 30 Good Good 822.79 2.1 315.24 Good 1-26 AM60 15 3 1 30Good Bad 1063.41 2.1 203.71 Bad 1-27 AZ31 0.1 3 2 30 Good Good 0.52 5.10.20 Good 1-28 AZ31 6 3 2 30 Good Good 200.60 2.9 76.86 Good 1-29 AZ3114.4 3 2 30 Good Good 837.70 2.1 320.95 Good 1-30 AZ31 15 3 1 30 GoodBad 1148.42 2.1 220.00 Bad 1-31 ZX10 0.1 3 2 30 Good Good 0.52 5.1 0.20Good 1-32 ZX10 6 3 2 30 Good Good 214.02 2.9 82.00 Good 1-33 ZX10 14.2 32 30 Good Good 850.13 2.1 325.71 Good 1-34 ZX10 15 3 1 30 Good Bad1219.52 2.1 233.62 Bad

TABLE 8 Electric Surface Treatment Coating film Galvanic Amount of ZrSample Alloy conductivity area ratio time Temperature Corrosion currentprecipitated No. type [mS/cm] [%] [minute] [° C.] resistance Adhesion[mA] pH [mg/m²] Appearance 2-1 AZ91 14.7 3 2 30 Good Good 696.88 2267.00 Good 2-2 AZ91 12 3 2 30 Good Good 360.19 2 138.00 Good 2-3 AZ91 73 2 30 Good Good 172.52 2.5 66.10 Good 2-4 AZ91 6 3 2 30 Good Good134.94 3 51.70 Good 2-5 AZ91 5 3 2 30 Good Good 97.62 4.5 37.40 Good 2-6AZ91 0.1 3 2 30 Good Good 0.29 6 0.11 Good 2-7 AZX911 0.1 3 2 30 GoodGood 0.29 6 0.11 Good 2-8 AZX911 6 3 2 30 Good Good 135.98 3 52.10 Good2-9 AZX911 14.7 3 2 30 Good Good 472.42 2 181.00 Good 2-10 AM60 0.1 3 230 Good Good 0.29 6 0.11 Good 2-11 AM60 6 3 2 30 Good Good 148.77 357.00 Good 2-12 AM60 14.5 3 2 30 Good Good 485.47 2 186.00 Good 2-13AZ31 0.1 3 2 30 Good Good 0.31 6 0.12 Good 2-14 AZ31 6 3 2 30 Good Good162.87 3 62.40 Good 2-15 AZ31 14.4 3 2 30 Good Good 501.13 2 192.00 Good2-16 ZX10 0.1 3 2 30 Good Good 0.31 6 0.12 Good 2-17 ZX10 6 3 2 30 GoodGood 176.70 3 67.70 Good 2-18 ZX10 14.2 3 2 30 Good Good 514.18 2 197.00Good

The surfaces of the Mg alloy member and the partner member included ineach of the samples were observed with a field emission scanningelectron microscope (FE-SEM). On the basis of the galvanic current, theamount of Zr precipitated, and the results of the observation, it wasconfirmed that, in the metal connection members of Sample Nos. 1-2 to1-8, 1-11 to 1-14, 1-16, 1-17, 1-19 to 1-21, 1-23 to 1-25, 1-27 to 1-29,and 1-31 to 1-33, a chemical conversion treatment film was formed on thesurfaces of the Mg alloy member and the partner member as a single piecewith covering the boundary between the two members. It was alsoconfirmed that, in the metal connection members of Sample Nos. 2-1 to2-18, a chemical conversion treatment film was formed on the surfaces ofthe Mg alloy member and the partner member as a single piece withcovering the boundary between the two members. In contrast, in the metalconnection members of Sample Nos. 1-1, 1-9, 1-10, 1-15, 1-18, 1-22,1-26, 1-30, and 1-34, a chemical conversion treatment film was notformed on the surfaces of the Mg alloy member and the partner member asa single piece with covering the boundary between the two members.

In particular, in Sample Nos. 1-5, 1-6, 1-12, 1-16, 1-20, 1-24, 2-3 to2-5, 2-8, 2-11, 2-14, and 2-17, where the amount of Zr precipitated was10 mg/m² or more and 100 mg/m² or less and the galvanic current was 200mA or less, appearance after the coating step was evaluated as good.

FIG. 2 is a graph illustrating the relationships between the chargetransfer resistance x₁ (Ω) of the Mg alloy member and the electricconductivity y (mS/cm) of the chemical conversion treatment liquid whichwere determined in Sample Nos. 1-2, 1-21, 1-25, 1-29, and 1-33. FIG. 3is a graph illustrating the relationships between the Al content x₂(mass %) in the Mg alloy member and the electric conductivity y (mS/cm)of the chemical conversion treatment liquid which were determined inSample Nos. 1-2, 1-21, 1-25, 1-29, and 1-33. As described in Tables 2 to7, each of Sample Nos. 1-2, 1-21, 1-25, 1-29, and 1-33 is the samplehaving the highest electric conductivity (mS/cm) among a group ofsamples having good corrosion resistance, good adhesion, and goodappearance and including the same type of alloy. The horizontal axes ofthe graphs illustrated in FIGS. 2 and 3 denote the charge transferresistance xi (Q) of the Mg alloy member and the A₁ content x₂ (mass %)in the Mg alloy member, respectively. The vertical axes of the graphsillustrated in FIGS. 2 and 3 denote the electric conductivity y (mS/cm)of the chemical conversion treatment liquid.

The linear approximation formulae determined from the graphs illustratedin FIGS. 2 and 3 (using Microsoft Excel, first order) were y=0.0007x₁+14.0 in FIG. 2 as denoted by the broken line and y=0.054 x₂+14.2 inFIG. 3 as denoted by the broken line. The intercept of the linearapproximation formula illustrated in FIG. 2 is an approximate valueobtained by rounding 13.967 to the first decimal place. The slope of thelinear approximation formula illustrated in FIG. 3 is an approximatevalue obtained by rounding 0.0542 to the third decimal place. Theintercept of the linear approximation formula illustrated in FIG. 3 isan approximate value obtained by rounding 14.208 to the first decimalplace. That is, it was found that a chemical conversion treatment filmcan be formed on the surfaces of the Mg alloy member and the partnermember as a single piece with covering the boundary between the twomembers while the two members are connected to each other in the casewhere at least one of the following conditions is satisfied: “Electricconductivity y of chemical conversion treatment liquid ≤5 0.0007×Chargetransfer resistance x₁+14.0” and “Electric conductivity y of chemicalconversion treatment liquid ≤0.054×Aluminum content x₂+14.2”. Thus, itwas found that, in the case where the Mg alloy member and the partnermember are connected to each other, the electric conductivity y of thechemical conversion treatment liquid needs to be adjusted in accordancewith at least one of the charge transfer resistance xi of the Mg alloyand the composition (in particular, the Al content x₂) of the Mg alloy.In addition, when the electric conductivity y of the chemical conversiontreatment liquid satisfies 0.1 mS/cm≤y, a chemical conversion treatmentfilm can be formed on the surfaces of the Mg alloy member and thepartner member as a single piece with covering the boundary between thetwo members while the two members are connected to each other, withoutconsuming an excessive amount of time.

In particular, it was also found that, when the electric conductivity yis 5.0 mS/cm or more and 6.0 mS/cm or less, the surface of the coatinghas good appearance. It was also found that a chemical conversiontreatment film can be formed on the surfaces of the Mg alloy member andthe partner member as a single piece with covering the boundary betweenthe two members while the two members are connected to each other whenthe pH of the chemical conversion treatment liquid is 2 or more and 6 orless and, in particular, when the pH of the chemical conversiontreatment liquid is 2.5 or more and 4.5 or less, the surface of thecoating material has good appearance.

It is intended that the scope of the present invention is not limited bythe above examples, is defined by the claims, and includes equivalentsof the claims and all modifications within the scope of the claims.

REFERENCE SIGNS LIST

1 METAL CONNECTION MEMBER

2 Mg ALLOY MEMBER

3 PARTNER MEMBER

4 CHEMICAL CONVERSION TREATMENT FILM

1. A metal connection member comprising: a Mg alloy member composedprincipally of magnesium; a partner member connected to the Mg alloymember, the partner member being composed principally of a metal noblerthan magnesium; and a chemical conversion treatment film that covers asurface of the Mg alloy member and a surface of the partner member withcovering a boundary between the Mg alloy member and the partner member.2. The metal connection member according to claim 1, wherein the chargetransfer resistance of the Mg alloy member in 1 M of Na₂SO₄ is 300 Ω ormore and 1200 Ω or less.
 3. The metal connection member according toclaim 1, wherein the content of aluminum in the Mg alloy member is 0.3%by mass or more and 12.0% by mass or less.
 4. A method for a chemicalconversion treatment of a metal connection member, the methodcomprising: a preparation step of preparing a metal connection member,the metal connection member including a Mg alloy member composedprincipally of magnesium and a partner member connected to the Mg alloymember, the partner member being composed principally of a metal noblerthan magnesium; and, a chemical conversion treatment step of bringingboth of the Mg alloy member and the partner member included in the metalconnection member into contact with a chemical conversion treatmentliquid in a collective manner to form a chemical conversion treatmentfilm on a surface of the metal connection member, wherein the electricconductivity y of the chemical conversion treatment liquid satisfies atleast one of the relation formulae (a) and (b) below,y≤0.0007 x ₁+14.0   (a)y≤0.054 x ₂+14.2   (b) where x₁ (Ω) is the charge transfer resistance ofthe Mg alloy member in 1M of Na₂SO₄, x₂ (mass %) is the content ofaluminum in the Mg alloy member, and y (mS/cm) is the electricconductivity of the chemical conversion treatment liquid.
 5. The methodfor a chemical conversion treatment of a metal connection memberaccording to claim 4, wherein the charge transfer resistance x₁ is 300 Ωor more and 1200 Ω or less.
 6. The method for a chemical conversiontreatment of a metal connection member according to claim 4, wherein thecontent of aluminum x₂ is 0.3% by mass or more and 12.0% by mass orless.
 7. The method for a chemical conversion treatment of a metalconnection member according to claim 4, wherein the electricconductivity y of the chemical conversion treatment liquid satisfies 0.1mS/cm≤y.
 8. The method for a chemical conversion treatment of a metalconnection member according to claim 4, wherein the content of zinc inthe Mg alloy member is 0.5% by mass or more and 6.2% by mass or less. 9.The method for a chemical conversion treatment of a metal connectionmember according to claim 4, wherein the pH of the chemical conversiontreatment liquid is 2.0 or more and 7.0 or less.
 10. The method for achemical conversion treatment of a metal connection member according toclaim 4, wherein the pH and the electric conductivity y of the chemicalconversion treatment liquid are adjusted using at least one acid or saltselected from nitric acid, sulfuric acid, hydrofluoric acid,hydrofluosilicic acid, bromic acid, manganic acid, permanganic acid,vanadic acid, hydrogen peroxide, an organic acid, and salts of theseacids.
 11. The method for a chemical conversion treatment of a metalconnection member according to claim 4, wherein the chemical conversiontreatment liquid includes an element belonging to Group 4 of theperiodic table.
 12. The method for a chemical conversion treatment of ametal connection member according to claim 4, wherein the temperature ofthe chemical conversion treatment liquid is 5° C. or more and 70° C. orless.
 13. The method for a chemical conversion treatment of a metalconnection member according to claim 4, wherein the Mg alloy member andthe partner member are electrically connected to each other.